HDU

版本一：

#include<bits/stdc++.h>
#define IO ios_base::sync_with_stdio(0);cin.tie(0);
using namespace std;
int prime[32770], phi[32770];
int main()
{IO;
    int T;
    cin >> T;
    while(T --){
        int n;
        cin >> n;
        memset(prime, 1, sizeof(prime));
        prime[0] = 0;
        prime[1] = 0;
        for(int i = 2; i * i <= n; i ++){
            if(prime[i]){
                for(int j = i + i; j <= n; j += i){
                    prime[j] = 0;
                }
            }
        }//找到所有n以内的素数
        for(int i = 1; i <= n; i ++)
            phi[i] = i;
        for(int i = 2; i <= n; i ++){
            if(prime[i]){
                for(int j = i; j <= n; j += i){
                    phi[j] = phi[j] / i * (i - 1);
                }
            }
        }//求出n以内所有数的欧拉值
        cout << phi[n] << endl;
    }
}

版本二：

#include<bits/stdc++.h>
#define IO ios_base::sync_with_stdio(0);cin.tie(0);
using namespace std;
int prime[32770], u[32770], phi[32770];
int main()
{IO;
    int T;
    cin >> T;
    while(T --){
        int n;
        cin >> n;
        int sum = 0;
        phi[1] = 1;
        memset(u, 1, sizeof(u));
        for(int i = 2; i <= n; i ++){
            if(u[i]){
                prime[++ sum] = i;
                phi[i] = i - 1;
            }
            for(int j = 1; j <= sum; j ++){
                if(i * prime[j] > n) break;
                u[i * prime[j]] = 0;
                if(i % prime[j]){
                   phi[i * prime[j]] = phi[i] * (prime[j] - 1);
                }
                else{
                    phi[i * prime[j]] = phi[i] * prime[j];
                }
            }
        }
        cout << phi[n] << endl;
    }
}